A defiant Abdullah Abdullah asserted that he had won Afghanistan’s bitterly disputed presidential election as outraged supporters threatened to back a breakaway government, raising the spectre of a prolonged and perhaps bloody political crisis.

A day after preliminary results gave a commanding lead to Abdullah’s rival, Ashraf Ghani, US officials were scrambling to salvage the legitimacy of an election that the Obama administration hoped would usher in a stable, post-American era in Afghanistan, but which has become tainted by accusations of serious fraud.

US President Barack Obama and Secretary of State John F. Kerry called Abdullah, urging him to abandon the idea of creating a “parallel government”. The US officials didn’t, however, threaten to cancel the proposed US-Afghan bilateral security agreement that both candidates have said they would support.

Kerry, on a two-day visit to China for strategic talks, plans to visit Kabul this week for further consultations, Abdullah told supporters. US officials said Kerry hasn’t yet made a decision on whether to stop in Kabul.

“We are the winner of the elections with no doubt,” Abdullah, a longtime opposition figure, told the supporters at an impassioned rally in Kabul. “We are not allowing a fraudulent government in this country even for one day.”

A few hours later, Ghani struck a measured tone, saying, “Our votes are clean.” He said he welcomed an audit of several thousand ballot boxes to be undertaken in the next two weeks. But the Afghan election bodies already suffer from a major credibility gap, and Western officials were worried that neither candidate would accept defeat.

The standoff grew tenser as Abdullah’s supporters tore down a picture of President Hamid Karzai at Kabul’s most prominent meeting hall and replaced it with one of Abdullah.

Abdullah later addressed the gathering and condemned the gesture, but repeated his allegations that Karzai’s presidential palace had conspired with election officials to rig the vote against him.

“If they abuse our loyalty and think that whatever they do, we will keep quiet, they are making a mistake,” he said. “They are making a historic mistake.”

Among Obama administration officials, there was growing concern that the crisis could undo many of the democratic strides Afghanistan has made in the 13 years since US forces invaded and toppled the Taliban government. All US forces are scheduled to depart the country at the end of the year, although an accord awaiting the approval of Afghanistan’s next president would allow 10,000 American troops to remain for an additional two years. Until fraud allegations surfaced in recent weeks, the election had been hailed as largely peaceful.

Kerry issued a statement late Monday expressing “grave concern” at the reports that a breakaway government could be formed. He urged Afghan election officials to investigate all fraud allegations thoroughly, but warned that any attempt to subvert the process could result in a cutoff of US and foreign assistance to Afghanistan, which provides nearly all of the government’s operating money.

“We call on all Afghan leaders to maintain calm in order to preserve the gains of the last decade and maintain the trust of the Afghan people,” Kerry said. “Any action to take power by extralegal means will cost Afghanistan the financial and security support of the United States and the international community.”

US officials said they believed the recount can be completed before the scheduled inauguration of the new president on August 2. They urged Abdullah and his supporters to be in touch with Ghani to begin trying to work out a unity government to hold the polarized country together.

The preliminary results from the June 14 runoff gave Ghani a lead of about 1 million votes out of 8 million cast, a remarkable turnaround from April’s first round of balloting, in which the former finance minister finished a distant second. Abdullah has released audio recordings purporting to show election officials working to rig votes in Ghani’s favour. One top election official resigned last month over the allegations but denied wrongdoing.

United Nations diplomats appeared to have brokered a deal late Monday under which both campaigns agreed to have 7000 out of 23,000 nationwide polling stations investigated for fraud. But Ghani’s supporters already have claimed victory, and Abdullah’s partisans on Tuesday were calling for their candidate to announce his Cabinet.

Amid the uncertainty, violence blamed on the Taliban has intensified, raising fear that insurgents could exploit the political chaos. Earlier Tuesday, a suicide bomber killed 16 people, including Afghan schoolchildren and four Czech soldiers, in eastern Afghanistan’s Parwan province.

The bombing took place near a school where soldiers from the US-led military coalition were handing out educational supplies to children, said Waheed Sediqqi, a spokesman for the provincial governor. About 7.30 am, a suicide attacker riding a bicycle detonated explosives near a crowd of international soldiers and civilians in the village of Qalandar Khail, killing the Czech service members, two Afghan police officers and 10 civilians, including children, Sediqqi said.

The US-led military coalition confirmed in a statement that four of its personnel were killed in an attack but gave no details.